High-pressure discharge lamps as well as highly loaded halogen incandescent lamps generate a relatively high proportion of UV radiation when the lamps operate. The lamp bulbs are made of quartz glass due the high thermal loading placed on the bulb. Quartz glass has a high degree of transparency for UV radiation in the range of between 400 nm to 200 nm. For many applications, the energy-rich UV radiation is undesirable, and may be harmful. UV radiation, in excess, has undesirable biological effects and, additionally, causes plastics and plastic components to become brittle. It is therefore necessary to reduce the transparency of the lamp bulb to UV radiation unless the lamps have an outer covering envelope which absorbs UV radiation.
The referenced U.S. Pat. No. 3,531,677, Loughridge, describes a high-pressure discharge lamp having a discharge vessel made of quartz glass. It is furnished with a UV absorbing coating or glaze. The UV absorbing coating is made of a eutectic mixture of Al.sub.2 O.sub.3 and SiO.sub.2. The eutectic coating is doped with between 0.05% to 10% of UV absorbing substances, for example TiO.sub.2 or CeO.sub.2.
The coating is made by providing a suspension of the Al.sub.2 O.sub.3 .multidot.SiO.sub.2 mixture and the UV absorbing substance, that is, either TiO.sub.2 or CeO.sub.2, in isopropyl alcohol with water. This suspension is sprayed on the bulb, dried, and then fired so that a glaze will result. It has been found that the UV transparency of such lamps is not reduced to a currently desired extent by the UV absorbing coating. The manufacturing process to so coat these bulbs, particularly drying and firing of the coating, is comparatively time-consuming and thus expensive for mass-produced lamp bulbs.
It has also been proposed to dope quartz glass directly when the glass is used for lamp bulbs, by doping the quartz glass with UV absorbing ions. This results in a reduction of the viscosity of the quartz glass, so that the thermal loading which can be placed on a quartz glass is reduced; this reduction also reduces the light output available from the lamp.